Antenna system

ABSTRACT

A method for scheduling radar transmissions for a phased array antenna system having at least two antenna faces, in which each respective antenna face is configured to receive echoes of possible targets corresponding to previous radar transmissions by the respective antenna face. The method includes determining, based on a list of requested radar transmissions, a scheduling process including starting times of radar transmissions for each of the antenna faces for substantially each radar transmission included in the list of requested radar transmissions. Further, the scheduling process is arranged such that the antenna faces are used substantially simultaneously, transmission by an antenna face is prevented when another antenna face is about to receive echoes corresponding to possible targets, and radar transmissions for each antenna face end at substantially the same time. In addition, radar transmissions for the antenna faces are substantially mutually synchronized.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method for scheduling radar transmissions fora phased array antenna system comprising at least two antenna faceswhere, after a radar transmission, each antenna face receives echoes,originating from that radar transmission, of possible targets and where,on the basis of a list of requested radar transmissions, an antenna faceand a starting moment are determined for at least substantially eachradar transmission.

2. Discussion of the Background

Phased array antenna systems as such are well-known in the art and arefor instance used on board naval ships for a considerable number oftasks which, until recently, required the availability of severalseparate fire-control and surveillance radar systems.

With multi-face phased array antennas, it is preferably assumed thatradar transmissions for the various antenna faces can be scheduledindependently. In this respect, a more specific assumption is thattransmission by one antenna face and reception by another antenna facemay coincide, or worded differently, that the insulation between thevarious antenna faces is satisfactory. Broadly speaking, this is true:it is possible to design the antenna faces such that the side lobe levelis kept to a minimum, thus providing a sufficient insulation for anycombination of beam directions.

The installation of the phased array antenna system on board a ship mayalter the situation. If a large object, a ferry-boat for instance, isirradiated by two different antenna faces simultaneously, which ispossible, as beams generated by two adjacent antenna faces willgenerally overlap to a slight extent, the infeasibility of coincidenttransmission and reception can be readily ascertained. This problem mayalso arise for less powerful reflectors and can be easily solved byscheduling the transmissions for adjacent antenna faces in such a mannerthat the beams are not parallel or not substantially parallel and bystipulating a predetermined frequency difference for transmissions foradjacent antenna faces. Notwithstanding these measures, the antenna faceside lobes and the presence of a powerful reflector in the vicinity ofthe antenna system are still found to cause crosstalk from one antennaface to another.

An ineffectual way to solve this problem is to use only one antenna faceat a time. This restricts the number of tasks to be performed by theantenna system to such an extent that the concept should no longer beconsidered viable.

SUMMARY OF THE INVENTION

The present invention provides a solution to the problem without anynoticeable degradation of the performance of the antenna system and ischaracterized in that the scheduling process is arranged such that theantenna faces are used at least substantially simultaneously and thattransmission by an antenna face at the moment that another antenna faceis about to receive echoes of possible targets is prevented.

According to a first embodiment of the invention, the prioritiesassigned to the radar transmissions may vary, which may be convenient inthe event of crosstalk or imminent crosstalk. It is theretocharacterized in that an antenna face engaged in a high-priority radartransmission may interrupt a lower-priority radar transmission performedby an adjacent antenna face at the moment that echoes of possibletargets are anticipated.

Crosstalk or imminent crosstalk may for instance be ascertained with theaid of a correlator, as described in the Netherlands patentspecification 1006812, which patent specification herewith isincorporated by reference.

An advantageous embodiment of the inventive method according to which itis not required to know the positions of possible targets ischaracterized in that radar transmissions for the various antenna facestake place at least substantially mutually synchronized.

Another advantageous embodiment of the inventive method, which all thetime ensures a maximum available listening time is characterized in thatradar transmissions for the various antenna faces are scheduled suchthat the radar transmissions end at least substantially simultaneously.

Yet another advantageous embodiment of the method is based on theobservation that a requested radar transmission has a certain degree offlexibility. It is not always necessary for the pulse repetitionfrequency, the PRF, to be chosen accurately, it will often suffice tostipulate that a PRF is contained in a certain frequency interval.Sometimes there may even be several frequency intervals that aresuitable. The same applies to the radar transmit frequency, the RF.Although the RP is often chosen dependent on the PRF with a view toDoppler processing, only a limited number of values is found to beunsuitable. Also the moment on which a transmission is to take place isnot entirely fixed, although in general there is an expiration time: themoment before which the transmission must be completed. This inventiveembodiment of the invention is therefore characterized in that eachrequested radar transmission is represented by a realization space whichcontains at least substantially all acceptable realizations of thetransmission. A priority assigned to the transmission and an expirationtime have been determined unequivocally and a large number of possiblePRF, RF pairs are available.

Yet another embodiment of the invention is characterized in that perantenna face, a group of requested radar transmissions, intended forthat antenna face, is selected from the list of requested radartransmissions on the basis of their priorities and that per group, therealizations of the requested radar transmissions are scheduled suchthat the requested radar transmissions can for all antenna faces berealized with an identical PRF at least substantially at any moment.

It frequently occurs that a requested radar transmission can be realizedby more than one antenna face, for instance because the range in azimuthof two adjacent antenna plates shows a certain degree of overlap.Usually, one realization of the requested radar transmission isnoticeably superior, sometimes, however, both realizations areacceptable. Yet another embodiment of the invention makes use of thispossibility and is characterized in that, if a group contains a radartransmission that can also be realized by another antenna face, thisradar transmission is included in the group pertaining to the otherantenna face after which, per group, the realizations of the requestedradar transmissions are scheduled such that the requested radartransmissions can for all antenna faces again be realized with anidentical PRF at least substantially at any moment.

Generally, a number of feasible time schedules will thus be generated,all of which are in fact suitable. Another advantageous embodiment ischaracterized in that from the variety of possible schedules, theschedule that ensures the most optimal utilization of all antenna facesis always selected.

Once all requested radar transmissions have been assigned to groups thusscheduled, the groups are written into a buffer memory for furtherprocessing.

A drawback in scheduling the groups is the absence of a good schedulingstrategy. Consequently, an absolutely optimal schedule is neverobtained. A longer search procedure will however yield a more feasibleschedule. Yet another advantageous embodiment of the invention ischaracterized in that the generation of possible schedules is ceasedwhen the buffer memory is at least substantially empty and that the bestfeasible schedule is selected from the schedules which are available atthat moment.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in greater detail with reference tothe following figures, of which:

FIG. 1 schematically represents a four-face phased array antenna system;

FIG. 2 schematically represents a control unit for a four-face phasedarray antenna system;

FIG. 3 schematically represents an alternative control unit for afour-face phased array antenna system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 schematically represents a phased array antenna system comprisingfour antenna faces 1, 2, 3, 4 suitable for shipboard applications andjointly providing a 360degree azimuthal coverage. Antenna faces 1, 2, 3,4 each receive input signals and control signals from a control unit 5and pass the echo signals received to a receiver unit 6 for furtherprocessing.

Antenna faces 1, 2, 3, 4 may be active or passive. Active antenna facesare obtained by an array of active T/R modules, each provided with an RFpower amplifier for the transmit function, an LNTA for the receivefunction and an adjustable phase shifter. Passive antenna faces areobtained by an array of passive T/R modules which usually only comprisean adjustable phase shifter.

Control unit 5 furthermore passes control signals to receiver unit 6,such that receiver unit 6 always knows when an antenna face is about toinitiate a certain type of transmission.

FIG. 2 schematically represents a radar control unit 5 incorporated in afour-face phased array antenna system.

Radar control unit 5 comprises four antenna face steerings 7.1, 7.2,7.3, 7.4, each of which controls one antenna face 1, 2, 3, 4. The actualsteering comprises a first connection via which a direction in azimuthand elevation for the antenna face in question is transmitted and asecond connection which passes a single RF pulse or a burst of RFpulses. Control commands for the antenna face steerings 7.i aresuccessively retrieved from a buffer memory 8 every time that allantenna face steerings have completed a preceding transmission.

According to the invention, all antenna face steerings employ the samePRF. In effect, this means that the control commands shall be availablein buffer memory 8 in an ordered way, such that the four controlcommands which are simultaneously retrieved from buffer memory 8 for thefour antenna faces actually have the same PRF. The antenna facesteerings are furthermore so designed that the trailing edges of the RFpulses emitted per antenna face will always coincide, for instance bytaking this joint trailing edge as reference point and by subsequentlytiming the pulses to be emitted from this point. This has the advantagethat the full listening time is available for each antenna face.

The control commands stored in buffer memory 8 and arranged according toPRF are thus sequentially processed by the antenna face beam steeringunits 7.1, 7.2, 7.3, 7.4. The buffer memory 8 is filled group by groupfrom a sort memory 9 in which the transmissions are sorted according toPRF, or more specifically, according to corresponding PRF realizations.Sort memory 9 in turn is filled from an input memory 10 in which theuser of the phased array radar system writes requested radartransmissions.

According to the invention, a requested radar transmission ischaracterized by a priority, an expiration time before which thetransmission shall be effected, a direction in azimuth and elevation, atype of transmission and at least one antenna face capable of realizingthe transmission. The type of transmission in turn defines at least onePRF range within which the PRF shall be realized, an associated RFrange, a pulse length and a number of pulses to be emitted. To simplifythe complex sorting process, a selection can be made from a number ofstandard transmissions, for instance:

1 pulse, 3, 10 or 100 microseconds

3 pulses, 3, 10 or 100 microseconds, PRF 1.8-2.2 KHz

16 pulses, 3, 10 or 100 microseconds, PRF 4.5-5.5 KHz

32 pulses, 3, 10 or 100 microseconds, PRF 9-11 KHz.

In addition to the PRF, an RF is per transmission specified by the userin a manner known with a view to avoiding blind speeds or, moregenerally, enabling a good Doppler processing. The RF need not beconsidered in the sorting process, although once a decision has beenmade in favour of a certain PRF realization, the RF will be determinedon the basis of this decision.

At the start of the sorting process, a group of requested transmissionsis selected from input memory 10 and written into sort memory 9,preferably a similar number per antenna face. The group is selected onthe basis of the priority of the requested transmissions and on theirexpiration times. The foremost aim is to process all transmissions froma certain priority onwards in due time. The relevant priority in thisrespect is determined on the basis of the instantaneous pressure. Therequested transmissions may already be available in input memory 10,ordered per antenna face, or they may during the transfer from inputmemory 10 to sort memory 9 be ordered for each antenna face. The resultis that at the start of the sorting process, the group is available, inthe form of four subgroups, one for each antenna face, in memories 9.1,9.2, 9.3, 9.4.

The sorting process is started with a certain random order in forinstance memory 9.1 and a procedure is started to find matchingrequested transmissions in the other memories 9.i, during whichcontinuous attempts are made to find PRF realizations such that the fourantenna faces transmit at the same PRF or, if so required, do nottransmit at all. Attempts will of course be made to create a situationin which all antenna faces can be active at practically any moment. Atthe end of the sorting process, sort memory 9 reveals a certain order onthe basis of which the utilization ratio of all antenna faces can beeasily ascertained.

Subsequently, if time permits, the requested transmissions areinterchanged in memory 9.1, for instance randomly, after which thesorting process is resumed. This may result in a more optimal order witha higher utilization ratio. This process may be repeated several times,each time retaining the most optimal order.

Several requested transmissions can be realized by more than one antennaface. It may then be advantageous to transfer such a transmission fromfor instance memory 9.i to memory 9.j and to repeat the sortingoperations several times as described above. This may result in a moreoptimal order.

By constantly retaining the most optimal order, an optimal scheduling ofrealizations of requested transmissions is always available at themoment that the buffer memory 8 threatens to become empty. Theserealizations constitute the new set of control commands with whichbuffer memory 8 is completed. Sort memory 9 is filled with a new groupof requested transmissions after which the sorting process can beresumed.

For the purpose of Doppler processing, a requested radar transmissionmade up of a number of pulse bursts is sometimes inevitable, each pulseburst having a slightly different, small PRF range. In addition, thebursts should follow each other in rapid succession. In that case, theabove-mentioned sorting process remains applicable to the fullestextent, although in changing the related bursts should remain mutuallycoupled.

FIG. 3 schematically represents an alternative radar control unit 5incorporated in a phased array antenna system. Radar control unit 5comprises an input memory 10 into which the user of the phased arrayantenna writes the requested radar transmissions per antenna face. Eachradar transmission is characterized by a priority, a direction inazimuth and elevation and a type of transmission. The type oftransmission in turn defines one PRF, one RF, a pulse length and anumber of pulses to be emitted. Additionally, a time gate may bedefined, i.e. a time interval measured from the trailing edge of acorresponding radar transmitter pulse in which a radar echo is expected.This is of particular importance for a radar transmission pertaining toa track process where the position of a target is approximately known.

Radar control unit 5 furthermore comprises a buffer memory 8 which, onthe basis of the requested radar transmissions, is filled from inputmemory 10. Buffer memory 8 is divided into the subbuffers 8.1, 8.2, 8.3,8.4, each of which controls one antenna steering 7.i. According to theinvention, subbuffer 8.1 also comprises a comparator circuit 11.1 which,at least for radar transmissions for which a time gate has been defined,compares the priority of the radar transmission to the priorities of theradar transmissions on the adjacent antenna faces. If the priorityassigned to the radar transmission exceeds that of a radar transmissionof an adjacent antenna face, the latter transmission is blanked for theduration of the time gate by means of blanking signals 12.1 and 13.1.Analogously, subbuffers 8.2, 8.3, 8,4 are likewise provided withcomparator circuits 11.2, 11.3, 11.4 which, if required, blank the radartransmissions of adjacent antenna faces.

This embodiment is particularly advantageous if the phased array antennasystem is designed to emit both long and short pulses, with the longpulses usually employed for search transmissions and the short pulsesfor track transmissions. By assigning a higher priority to a tracktransmission than to a search transmission, the crosstalk-free receptionof echoes originating from a track transmission can be ensured, whereasthe adverse effect of a brief interruption of a long search transmissionis negligible.

What is claimed is:
 1. A method for scheduling radar transmissions for aphased array antenna system having at least two antenna faces, eachrespective antenna face of the at least two antenna faces configured toreceive echoes of possible targets corresponding to previous radartransmissions by said each respective antenna face, and said methodcomprising: determining, based on a list of requested radartransmissions, a scheduling process including starting times of radartransmissions for each of the at least two antenna faces forsubstantially each radar transmission included in the list of requestedradar transmissions, wherein the scheduling process is arranged suchthat the at least two antenna faces are used substantiallysimultaneously, transmission by a first antenna face of the at least twoantenna faces is prevented when a second antenna face of the at leasttwo antenna faces is about to receive echoes corresponding to possibletargets, and trailing edges of first and second radar transmissions foreach of the at least two antenna faces are aligned so the first andsecond radar transmissions end at substantially the same time even whenthe first and second radar transmissions begin at different times, andwherein radar transmissions for the at least two antenna faces aresubstantially mutually synchronized.
 2. The method as claimed in claim1, wherein an antenna face engaged in a high-priority radar transmissioninterrupts a lower-priority radar transmission preformed by an adjacentantenna face when echoes of possible targets are anticipated to bereceived by the antenna face engaged in the high-priority radartransmission.
 3. The method as claimed in claim 1, wherein transmissionparameters for performing each requested radar transmission included inthe list of requested radar transmissions are flexibly determined fromall acceptable performance parameters.
 4. The method as claimed in claim3, wherein per respective antenna face, a group of requested radartransmissions intended for the respective antenna face is selected fromthe list of requested radar transmissions based on priorities of therequested radar transmissions, and that per group, the transmissionparameters for the requested radar transmissions are determined andscheduled so the radar transmissions for the at least two antenna faceshave a same pulse repetition frequency.
 5. The method as claimed inclaim 4, wherein, if a group contains a radar transmission that can alsobe performed by another antenna face, the radar transmission istransferred to the group pertaining to the other antenna face afterwhich, per group, the transmission parameters for the requested radartransmissions can again be determined and scheduled so the radartransmissions for the at least two antenna faces have a same pulserepetition frequency.
 6. The method as claimed in claims 4 or 5, whereinfrom a variety of possible schedules, a schedule that ensures the mostoptimal utilization of all antenna faces is always selected.
 7. Themethod as claimed in claim 6, wherein the scheduled groups are writteninto a buffer memory included in the phased array antenna system forfurther processing.
 8. The method as claimed in claim 7, wherein ageneration of possible schedules is stopped when the buffer memory issubstantially empty and a best feasible schedule is selected from thegeneration of possible schedules.